What are Digital signals and Analog signals — advantages of digital methods
ANALOG SIGNALS
We are very familiar with analog signals. The reading of a moving coil or moving iron voltmeter and ammeter, dynamometer wattmeter, etc., are all analog quantities. The trace on a CRO screen is also analog.
Analog methods for communication system have long been in use. Frequency division multiplexing is the means of analog communication. An electronic amplifier is an analog circuit.
The low-level analog signal (audio, video, etc.) is amplified to provide strength to the signal. Analog circuit systems (position control, process control) have been in use for the past many decades. Analog Computers use voltages, resistances and potentiometric rotations to represent the numbers and perform arithmetic operations. Analog differentiation, integration, etc., is also done.
An operational amplifier is a very versatile analog electronic circuit used to perform a variety of operations (addition, subtraction, multiplication, division, exponentiation, differentiation, integration, etc.). Analog integrated circuits are widely used in electronic industry.
DIGITAL SIGNALS
The term digital is derived from digits. Any device or system which works on digits is a digital device or system. A digital voltmeter indicates the value of voltage in the form of digits, e.g., 230.25. Reading an analog instrument introduces human error and also requires more time. Digital reading is more accurate, eliminates human error and can be read quickly.
Communication systems have also gone digital. The initial signal waveform is always analog. To use digital transmission, the signal waveform is sampled and the digital representation transmitted. The process of converting an analog signal to digital form is also known as digitizing. For multiple channels of transmission, Time Division Multiplexing is used.
Digital control systems are fast replacing analog control systems. In digital control systems, the error is in the form of digital pulses.
Digital computers have revolutionalized the concept of computers. Their capability ranges from simple calculations to complex calculations using numerical techniques. Many computing tasks which required hours and days take only a few minutes on digital computers.
Digital signal processing is concerned with the representation of continuous-time (analog) signals in digital form. It is based on Claude Shannon’s (Sampling is done to convert analog signal to digital signal.) sampling theorem which states that “A band-limited continuous-time signal can be reconstructed in its entirety from a sequence of samples taken at intervals of less than 1/2fN where fN is the highest frequency present in the signal.” It is essential that the analog signal is band-limited which limits how much it can change between samples. The sampling rate was too high to ensure accuracy.
Since the initial signal is always analog and the final required signal is also mostly analog, a digital system requires three essential aspects (1) conversion of an analog signal to digital form (2) transmission of digital signal (3) reconstruction of an analog signal from the received digital signal as shown in Fig. 1.1
A continuous-time function x(t) is converted into a digital signal x(n) by an analog to digital (A/D) converter. The output of a discrete time system is y(n) and is converted to continuous time function by digital to analog (D/A) converter. The discrete-time system, in digital communications, is a digital communication channel. To achieve high fidelity, the sampling rate may have to very high say 50000 samples per second. Each sample may be encoded by (say) 18 bits. The frequency fs (in Fig. 1.1) must be more than twice fN the highest frequency in the analog signal. Very large scale integration (VLSI) digital circuits have the capability to sample at a very fast rate so that high fidelity is achieved.
A DSP (digital signal processing) chip is the core of a digital system used in cellular phones, modems, disk drives, digital automotive systems, etc. It was invented only about 15 years ago but its applications have grown tremendously.
Digital methods have the following advantages over analog methods
- Digital devices work only in two states (say on and off). Thus their operation is very simple and reliable.
- Digital display is very accurate and can be read at a fast speed. Human error is eliminated.
- Electronic components exhibit a change in behaviour due to ageing, change of ambient temperature, etc. Therefore, the behaviour of analog circuits tends to be somewhat unpredictable. However, digital circuits are free from these defects.
- Digital ICs are very cheap and compact in size.
- Variety of digital ICs are available.
- Power requirement of digital circuits is very low.
- Digital systems have the characteristic advantage of memory. Thus, information can be stored over a period of time. The space required for this stage is very small. One compact disc (CD) can store information contained in many books.
- Digital systems have high fidelity and provide noise-free operations.
- By integrating system peripheral functions on a DSP chip, the reliability can be enhanced and cost reduced.
- When volumes are high, they can be manufactured at low cost.
- The same digital system can be used with a variety of software for a number of tasks.
- Standardization & Repeatability.
